This application claims priority to an application entitled xe2x80x9cRESIDUAL STRESS MEASURING DEVICE FOR OPTICAL FIBER,xe2x80x9d filed in the Korean Industrial Property Office on Mar. 18, 2002 and assigned Serial No. 2002-14509, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an optical fiber, and more particularly, to a device for measuring residual stress of an optical fiber.
2. Description of the Related Art
It is a common during the course of fabricating an optical fiber that a part of stress does not get completely removed from the optical fiber. This type of undesirable stress in the optical fiber is known as residual stress. As such, the measurement of residual stress is an essential element in the production of high-quality optical fibers.
The residual stress that is typically generated during the fetch process, which performed at a high temperature as one phase of the production process of an optical fiber, increases light loss due to the light scattering of the optical fiber and causes the refractive index to change because of the photoelastic phenomenon. Therefore, in order to fabricate a high-quality optical fiber, more research efforts must be given to the development of optical fiber production (fetch) technologies for suppressing the residual stress. At the same time, it is also necessary to develop appropriate equipments for performing a precise measurement of the residual stress in an optical fiber.
As an attempt to release some of the residual stress in an optical fiber, a long-period fiber grating has been under development. In order to study the spectrum transmittance characteristic of such element and to perform researches on the improvement of such characteristics, any changes in the refractive index due to the change in the residual stress as well as the distribution of the residual stress along the longitudinal direction of the optical fiber must be measured on a regular basis. Particularly, in-depth studies should be done in connection with the changes in the refractive index due to the residual stressxe2x80x94that is, the changes in the distribution of doping material under the influence of photoelastic effect and a nonlinear phenomenon. To implement such studies, however, a special kind of measurement device, which is capable of making three-dimensional measurement and observing of the residual stress of an optical fiber and its photoelastic effect, is absolutely required.
In general, the measurement of the residual stress in an optical fiber or in an optical fiber preform is achieved using the photoelastic effect. The photoelastic effect relates a phenomenon that the refractive index of a medium changes in accordance with the direction of the residual stress remaining in the medium. Because of this photoelastic effect, the refractive index of an optical fiber or its preform tend to change according to a polarizing direction of light.
The measurement of the residual stress in an optical fiber was first introduced in the Journal of P. L. Chu and T. Whitebread, xe2x80x9cMeasurement of stresses in optical fiber and preform, Appl. Opt. 1982, 21, pp. 4241xcx9c4245xe2x80x9d. In the Journal, a number of studies have been performed on the measurement and its related measuring device. Further, a method for calculating residual stresses in an optical fiber through the phase changes of a transmitted light by photoelastic effect is already disclosed in the U.S. patent application Ser. No. 09/803,873, filed on Mar. 13, 2001 by the Applicant.
FIG. 1 is an explanatory diagram of the device for measuring the residual stresses in an optical fiber according to a related art. As depicted in the drawing, the residual stress measuring device for an optical fiber 10 includes a light generator 11, a lens system 13, a reflecting mirror 15, a focusing lens unit 17a, an objective lens 17b, and a detector 19. In operation, an optical fiber 30b with residual stresses is fixated on a designated flat plate 30a, and the plate is disposed between the focusing lens unit 17a and the objective lens 17b. 
The light outputted from the light generator 11 passes through the lens system 13, then converted to a plane wave, which in turn incidents on the optical fiber 30b through the reflecting mirror 15 and the focusing lens unit 17a. Then, the phase of the incident light upon the optical fiber 30b changes depending on whether or not there is residual stress in the optical fiber. To calculate the residual stress, the detector 19 detects a polarized light towards a certain direction from the objective lens 17b. 
However, the above-described method, i.e., measuring the residual stress in the optical fiber fixated on the flat plate, can measure the residual stress from one direction only. Therefore, to be able to measure the residual stress in other directions, the optical fiber should be reset toward the direction according to the desired measurement direction. As a result, the frequent changes in the experiment conditions tend to cause an erroneous measurement of the residual stress.
Accordingly, the conventional method has drawbacks in that it makes difficult to perform a three-dimensional measurement of the distribution of residual stress and as well as the measurement of asymmetry of the distribution of the residual stress in an optical fiber.
The present invention overcomes the above-described problems, and provides additional advantages, by providing a method and apparatus for measuring a residual stress in an optical fiber that is capable of obtaining a more accurate distribution of the residual stress in the optical fiber.
According to an embodiment of the present invention, a device for measuring a residual stress of an optical fiber includes: a fixating unit for fixating an optical fiber having residual stresses to be measured therein; a measuring unit comprising a light generator for generating a light for measuring the residual stresses in the optical fiber and a detector for detecting phase changes of the light that has been generated by the light generator; wherein the measuring unit measures the residual stresses in the optical fiber based on the phase changes of the light passed through the optical fiber, while the measuring unit is rotating along with the circumference of the optical fiber.